Molten aluminum and molten alkali metals and vapors are known to attack refractory walls and linings made of alumina, silica, magnesia, magnesite, chromite, and other materials. Prolonged exposure to these molten metals and vapors promotes corrosion and degradation of various parts of furnaces and other containment vessels, including walls, ceilings, roofs, exhaust ducts, floors, ramps, skim shelves, spouts, tap holes, troughs, runners, launders, lentils, door jams, and doors. In the past, this required periodic replacement of the refractory linings and/or the containment vessels. This periodic replacement often entailed significant down time and expense.
Only a small amount of corrosion and degradation is required to adversely affect the thermal insulation properties of the refractory wall or lining. When the refractory wall or lining is formed of alumina-silicate, as is commonly the case, the molten aluminum chemically reacts with the refractory to form corundum, which is a mixture of Al2O3 with unreacted Si and Al. This reaction product strongly attaches itself to the refractory by filling its porosity, and is very difficult to remove.
By way of example, molten aluminum reacts with silica in the refractory wall or lining as follows:4Al+3SiO2→2Al2O3+3Si
If the molten aluminum is a metal-bearing alloy, such as an aluminum-magnesium alloy, the following additional reactions may occur:2Mg+SiO2→2MgO+Si3Mg+4Al2O3→3MgAl2O4+2Al
Other oxides frequently found in refractories are also reduced by reaction with molten aluminum and alkali metals and vapor. These include oxides of titanium and iron, for example. There is a need or desire for an easy-to-apply coating that protects refractory walls and linings from chemical attack by molten aluminum and alkali metals and vapors.